Does splitting up an active record query over 2 methods hit the database twice? - ruby-on-rails

Question

I have a database query where I want to get an array of Users that are distinct for the set:
#range is a predefinded date range
#shift_list is a list of filtered shifts
def listing
Shift
.where(date: #range, shiftname: #shift_list)
.select(:user_id)
.distinct
.map { |id| User.find( id.user_id ) }
.sort
end
and I read somewhere that for readability, or isolating for testing, or code reuse, you could split this into seperate methods:
def listing
shiftlist
.select(:user_id)
.distinct
.map { |id| User.find( id.user_id ) }
.sort
end
def shift_list
Shift
.where(date: #range, shiftname: #shift_list)
end
So I rewrote this and some other code, and now the page takes 4 times as long to load.
My question is, does this type of method splitting cause the database to be hit twice? Or is it something that I did elsewhere?
And I'd love a suggestion to improve the efficiency of this code.
Further to the need to remove mapping from the code, this shift list is being created with the following code:
def _month_shift_list
Shift
.select(:shiftname)
.distinct
.where(date: #range)
.map {|x| x.shiftname }
end
My intention is to create an array of shiftnames as strings.
I am obviously missing some key understanding in database access, as this method is clearly creating part of the problem.
And I think I have found the solution to this with the following:
def month_shift_list
Shift.
.where(date: #range)
.pluck(:shiftname)
.uniq
end

Answer 1

Nope, the database will not be hit twice. The queries in both methods are lazy loaded. The issue you have with the slow page load times is because the map function now has to do multiple finds which translates to multiple SELECT from the DB. You can re-write your query to this:
def listing
User.
joins(:shift).
merge(Shift.where(date: #range, shiftname: #shift_list).
uniq.
sort
end
This has just one hit to the DB and will be much faster and should produce the same result as above.
The assumption here is that there is a has_one/has_many relationship on the User model for Shifts
class User < ActiveRecord::Base
has_one :shift
end
If you don't want to establish the has_one/has_many relationship on User, you can re-write it to:
def listing
User.
joins("INNER JOIN shifts on shifts.user_id = users.id").
merge(Shift.where(date: #range, shiftname: #shift_list).
uniq.
sort
end
ALTERNATIVE:
You can use 2 queries if you experience issues with using ActiveRecord#merge.
def listing
user_ids = Shift.where(date: #range, shiftname: #shift_list).uniq.pluck(:user_id).sort
User.find(user_ids)
end